Universal Windows Platform apps

Left: Traditional desktop app; elaborate and crowded even without contents
Right: Metro-style app; chiefly composed of contents only

Metro-style apps, officially known as Windows Store apps, is a class of mobile apps distributed via Windows Store for Microsoft Windows. These apps are unlike traditional Windows desktop software in terms of design, development, distribution and contents.

Look and feel

Found in Windows, Mac OS and Linux, traditional desktop software could run inside windows with distinct borders. Their border could bear a title bar which displayed application title, a system menu, and optionally a set of maximize, minimize, close and help buttons. Their window could be resized horizontally and vertically. They are controlled by visible elements of user interface, including menus, toolbars, ribbons, scrollbars and window borders, which have become so elaborate in Windows Vista and Windows 7 that require specialized apps (such as Window Clippings)for taking screenshots.

Metro-style apps do not run in windows. Either they occupy the entire screen or are snapped to one side, in which case they occupy the entire breath of screen but only part of its width. True to Microsoft's "Contents are the hero" slogan, Metro-style apps have no title bar, system menu or window control buttons. Even mandatory command interfaces like scrollbars are usually hidden at first. Metro-style apps do not have menus on their own windows; rather, elaborate menu are located on Settings charm.

Distribution and licensing

Traditionally, Windows was an open operating system. Software could come from anywhere, including optical discs manufactured by their publishers and downloaded packages from the Internet. One could potentially run such app without the knowledge of any other soul in the world. Only components created by publisher are in charge of licensing, updating, maintaining or uninstalling them. Publishers have a free hand in choosing the licensing terms.

For most users, the only point of entry of Metro-style apps is Windows Store, which requires users to have an Internet connection and a Microsoft account. Thus, Microsoft always knows which apps any given user runs. Outside the 120 countries in which Windows Store works, users can neither develop nor have such apps. The publisher is always Microsoft, and Windows Store Service is the component in charge of installing, updating or uninstalling these apps. Metro-style apps may disappear from the user's computer if Microsoft so wishes. Also, Microsoft specifies the licensing terms. More specifically, developers must allow users to run their purchased applications on multiple devices. Enterprises operating a Windows domain infrastructure may enter a contract with Microsoft that allows them to sideload their line-of-business Metro-style apps, circumventing Windows Store.

Metro-style apps are the only third-party apps that run on Windows RT. Traditional third-party apps do not run on this operating system.

Multitasking

Before Windows 8, computer programs were identified by their static computer icons. Windows taskbar was responsible for representing every app that had a window when they run. Metro-style apps, however, are identified by their "tile"s that can not only show their icon, but also other dynamic contents. They do not show up on Windows taskbar when they run. Instead, they appear on a dedicated app switcher on the left side of the screen.

Lifecycle

Before Metro-style apps, application software on personal computers were started and ended with user command. Metro-style apps, however, never truly start or end on user's command. Since they have dynamic tiles, they are running even before the user has requested. In addition, issuing a close command does not end their process or free the memory they have occupied: A Windows app manager automatically stops and terminates hung, inactive and closed apps independent of user's wish.

Development

Traditionally, software were developed using Windows API (also known as win32). Software had access to the API with no arbitrary restrictions. Developers were free to choose their own programming language and development tools. Metro-style apps, however, are developed using Windows Runtime (WinRT). Calling forbidden API disqualifies the app from appearing on Windows Store.

Metro-style apps only be developed using Microsoft's own development tools. Despite claims of interoperability between multiple languages, a practical attempt to implement independent binding to Windows Runtime for a native language is currently doomed to fail. According to Allen Bauer, Chief Scientist of Embarcadero Technologies, there are APIs that every computer program must call but Microsoft has forbidden them, except when the call come from Microsoft's own Visual C++ runtime.

Security

Windows software have unlimited power to use and change their ecosystem however they want. Windows user account rights, User Account Control and antivirus software attempt to keep this ability in check but do not prevent it. Metro-style apps, however, are sandboxed and cannot permanently change a Windows ecosystem. Microsoft further moderates these apps and may remove them if they are discovered to have security or privacy issues.